1. Field of the Invention
The present invention relates to signaling in a Multi User-Multiple Input Multiple Output (MU-MIMO) wireless communication system. More particularly, the present invention relates to an apparatus and method for providing Downlink Control Information (DCI) in a MU-MIMO wireless communication system.
2. Description of the Related Art
The rapid growth of the wireless mobile communication market has resulted in a greater demand for various multimedia services in a wireless environment. Recently, to provide such multimedia services, which include a large amount of transmit data and increased data delivery rate, research is being conducted on Multiple Input Multiple Output (MIMO) wireless communication systems that provide a more efficient use of limited frequencies.
A MIMO wireless communication system can transmit a signal over independent channels per antenna and thus increase transmission reliability and data throughput without the use of an additional frequency or need for additional transmit power, as compared to a single-input single-output system. Furthermore, the MIMO wireless communication system can be extended to a MIMO system in a Multi User (MU) environment supporting a plurality of users. Such an MU-MIMO system enables the plurality of users to share spatial resources ensured by the multiple antennas, thus further improving the spectral efficiency.
In the next generation communication system employing MU-MIMO, research is actively in progress to provide a variety of Quality of Services (QoS) with a data transfer speed of about 100 Mbps. Representative examples of such communication systems include the Institute of Electrical and Electronics Engineers (IEEE) 802.16 system and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard. Both the IEEE 802.16 system and the LTE standard employ Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme so that a broadband network can be supported in a physical channel.
FIGS. 1A and 1B illustrate a generic downlink frame structure used in a wireless communication system employing OFDM according to the related art.
Referring to FIG. 1A, the generic frame structure used in OFDM downlink includes a 10 ms radio frame 101 divided into 20 equal slots 103 of 0.5 ms. A sub-frame 105 consists of two consecutive slots such that one frame includes 10 sub-frames.
Referring to FIG. 1B, a generic structure of a resource grid for the duration of one downlink slot 103 is illustrated. The available downlink bandwidth consists of NBWDL sub-carriers with a spacing of 15 kHz. The value of NBWDL can vary in order to allow for scalable bandwidth operation up to 20 MHz. One downlink slot also consists of NSymbDL symbols, each symbol including a Cyclic Prefix (CP) added as a guard time such that the value of NSymbDL depends on the length of the CP. As illustrated in FIG. 1, the generic frame structure with normal CP length has NSymbDL=7 symbols.
In a wireless communication system employing OFDM technology, data is allocated to a Mobile Station (MS) using Resource Elements (REs) 107 of a resource block 109. As illustrated in FIG. 1B, a resource block 109 consists of 12 consecutive sub-carriers in the frequency domain and NSymbDL consecutive symbols in the time domain. Depending on the required data rate, each MS can be assigned one or more resource blocks in each transmission interval of 1 ms (i.e., 2 slots or 1 sub-frame), the resource assignment being performed by a Base Station (BS). The user data is carried on a Physical Downlink Shared Channel (PDSCH) and the downlink control signaling, used to convey scheduling decisions to individual MSs, is carried on the Physical Downlink Control Channel (PDCCH). The PDCCH is located in the first OFDM symbols of a slot.
An aspect of the OFDM technology is the use of reference signals that are provided within the resource blocks for each MS. The reference signals are used by an MS for cell search, channel estimation, neighbor cell monitoring, mobility measurements, and the like. Moreover, the types of reference signals include a Cell-specific Reference Signal (CRS) and an MS specific reference signal, also known as a Dedicated Reference Signal (DRS).
FIGS. 2A through 2G illustrate downlink CRSs used in 1-antenna, 2-antenna and 4-antenna configurations according to the related art.
Referring to FIGS. 2A through 2G, pre-defined REs are used to carry the CRS sequences depending on the number of antennas. In the single antenna system illustrated in FIG. 2A, a CRS is placed in the RE associated with the #0 and #4 symbols of each slot in the time domain. In the frequency domain, the CRS is placed in the RE associated with each 6th subcarrier, there being a staggering of 3 subcarriers between symbols. In the two and four antenna systems of FIGS. 2B through 2G, CRSs are placed in REs in a fashion similar to that of the single antenna system, there being an offset of 3 subcarriers between CRSs for the different antennas. Moreover, with reference to the 2-antenna system (FIGS. 2B and 2C) and 4-antenna system (FIGS. 2D through 2G), REs used for CRS transmission of one antenna are not used for transmission on the other antenna(s).
FIG. 3 illustrates a downlink DRS for use in a wireless communication system employing OFDM technology according to the related art.
Referring to FIG. 3, a DRS pattern, indicated by elements (R5), is illustrated in a pair of resource blocks along with unnumbered CRSs of a 4-antenna system. In contrast to the CRS, which uses 8 REs per resource block pair, the DRS uses 12 REs within the pair of resource blocks. The DRSs are supported for 1-antenna transmission of PDSCH and the MS is informed by a higher layer as to whether the DRS is present. Moreover, the DRS is transmitted only on the resource blocks upon which the corresponding PDSCH is mapped, the PDSCH and antenna port using the same pre-coding.
The downlink control signaling, used to convey scheduling decisions to individual MSs, is carried on the PDCCH, which is located in the first OFDM symbols of a slot. The information carried on the PDCCH is referred to as Downlink Control Information (DCI). Depending on the purpose of the control message, different formats of DCI are defined. More specifically, the 3GPP Technical Specification (TS) 36.212 defines various formats of DCI based on different needs of the communication system at the time of scheduling. For example, DCI Format 0 is used for the scheduling of a Physical Uplink Shared Channel (PUSCH), and DCI Format 1 is used for the scheduling of one PDSCH codeword. In TS 36.212, there are 10 DCI formats (i.e., formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 3 and 3A), each DCI format including various information that may be used in conjunction with the reference signals for receiving data transmitted by the BS.
As the technology regarding wireless communication systems continue to advance, improvements are being made regarding transmission and reception of greater amounts of data. These improvements often require additional or different control information to be transmitted from a BS to an MS. Accordingly, there is a need for an improved apparatus and method for providing and using control information in a wireless communication system.